there isn't a really quick way mate, you have to remove the wheel, lift the caliper up and take the brake pads out, then take the brake disc off, once done it's just the hub bolt then pull the whole bearing off, then you can measure the width of the stub axle or measure the internal diameter of the old bearing, it's not a big job, won't even take you a hour. but you'll need that special socket to remove the hub bolt.